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    SubjectRe: [Xen-devel] Regressions in v3.3-rc1 introduced by "xen/granttable: Grant tables V2 implementation"
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    On Wed, 2012-01-25 at 20:50 +0000, Konrad Rzeszutek Wilk wrote:
    > On Wed, Jan 25, 2012 at 03:23:28PM +0000, Paul Durrant wrote:
    > > > -----Original Message-----
    > > > From: Paul Durrant
    > > > Sent: 25 January 2012 07:17
    > > > To: Ian Campbell; Konrad Rzeszutek Wilk
    > > > Cc: annie.li@oracle.com; xen-devel@lists.xensource.com; linux-
    > > > kernel@vger.kernel.org
    > > > Subject: RE: [Xen-devel] Regressions in v3.3-rc1 introduced by
    > > > "xen/granttable: Grant tables V2 implementation"
    > > >
    > > > > -----Original Message-----
    > > > >
    > > > > On HVM for gnttab_shared we make an add_to_physmap call with
    > > > > XENMAPSPACE_grant_table (in gnttab_map). I don't see any support call
    > > > > to do something similar for the status array though and I don't see a
    > > > > XENMAPSPACE_* specifically for that case either in the hypervisor either.
    > > > >
    > > >
    > > > There is no map-space for the status pages. To map them you use the
    > > > standard map space but OR a bit (top bit IIRC) into in the idx. It's a bit icky,
    > > > but that's how it is.
    > > >
    > >
    > > I fixed a bug in xen-unstable a few weeks back that prevented
    > mapping of the status frames so I guess the bug is possibly due to
    > trying to use gnttab 2 on 4.1, where this bug would hit, but failing
    > to check the return code from the status mapping hypercall?
    >
    > changeset: 24428:5b4b7e565ab8
    > user: Paul Durrant <paul.durrant@citrix.com>
    > date: Sun Dec 18 14:39:14 2011 +0000
    > summary: Fix grant_table v2 status mapping.
    >
    > changeset: 24427:931bf1105730
    > user: Paul Durrant <paul.durrant@citrix.com>
    > date: Sun Dec 18 14:38:32 2011 +0000
    > summary: Allow VMs to query their own grant table version.
    >
    >
    > Looks like c/s 24427 needs to be in the hypervior. Hm, but that exists
    > in 4.1 too as 23204. Will try back-porting cs 24428 in Xen 4.1 and seeing
    > if that does it.

    I think this bug is a red herring -- the current code in Linux will
    never get anywhere near triggering it because it never even tries to map
    the status pages into the p2m.

    IOW the call to XENMEM_add_to_physmap in
    drivers/xen/grant-table.c:gnttab_map() needs to be done for the status
    pages too or there is no chance of v2 grant tables working for HVM
    guests.

    I think the best bet is to disable v2 for HVM guests for now (something
    like your second patch) and implement it for 3.4.

    Ian.




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